This invention relates to amplification devices, in particular, amplification devices connected to signal sources with high internal impedances.
Vibration sensors are sensors for detecting forces exerted on an object to be measured, in other words, for detecting accelerations of the object, and for converting the magnitudes of the forces into electric signals. The range of uses of vibration sensors is wide, which include, for example: monitoring of abnormal vibration in motors etc.; control of a balance of lathe etc.; monitoring of conditions of air conditioners etc.; measurements of characteristics of HDDs etc.; medical diagnoses and treatments using ultrasonically vibration etc.; hand movement detection for digital video cameras etc.; and sound detection for cellular phones etc. In many of those uses, both of miniaturization and high reliability are demanded of vibration sensors.
The vibration sensors of miniature size widely used in general are of the piezoelectric types, which use piezoelectric devices. In addition, especially as sound sensors, capacitive microphones such as electret condenser microphones (ECM) are frequently used. Such vibration sensors, in general, have high internal impedances, which are determined mainly by their capacitive reactance components; for example, a piezoelectric device has an equivalent capacitance within the range from several picofarads to several tens of picofarads, in general. Therefore, the internal impedances are especially high in the low frequencies.
The output signals of vibration sensors are amplified at a rate within the range from a few tens to several hundreds of times, in many cases. Furthermore, for uses of vibration sensors with particular emphasis on the low frequencies of the output signals such as sound detection, in general, the lower limit of the frequencies of the output signals to be amplified is usually set at the order of 10 Hz. For such amplifications, in general, multistage amplification devices are widely used. In particular, the first stage of the amplification device is connected directly to a vibration sensor. The first stage of the amplification device requires a high input impedance, when the vibration sensor has a high internal impedance. In an amplification device with a high input impedance, its bias circuit requires some contrivance. In a conventional amplification device 100 connected to a piezoelectric vibration sensor acting as a signal source, for example, a bias circuit 20 is configured as follows. See FIG. 7. In this bias circuit 20, two resistors 2H and 2L are connected in series between a power supply terminal VDD and a ground terminal, thereby constituting a voltage divider. The node J between the two resistors 2H and 2L, that is, the output terminal J of the voltage divider 2V is connected through a third resistor 2R to an input terminal IN of the operational amplifier 1. The resistance value of the third resistor 2R is set at a value far higher than any of the resistance values of the two resistors 2H and 2L. Furthermore, the operational amplifier 1 has a high input impedance because of its MOSFET input stage. Accordingly, direct currents hardly flow in the third resistor 2R, and thus, voltage drops hardly occur across the third resistor 2R. Therefore, the potential of the output terminal J of the voltage divider 2V is applied to the input terminal IN of the operational amplifier 1 for use as a bias voltage. Thereby, the output terminal OUT of the operational amplifier 1 is maintained at a potential a predetermined-ratio times as high as the potential of the input terminal IN. In FIG. 7, the potential of the output terminal OUT is equal to the potential of the input terminal IN, since the operational amplifier 1 is of a voltage-follower type.
Especially for the alternating signals provided from the piezoelectric device 3, the voltage divider 2V acts as a constant voltage source. See FIG. 8. The internal impedance Z of the piezoelectric device 3 is determined mainly by its capacitive reactance component. Actually, the internal impedance Z is expressed by the following equation:Z=1/(2πfC),where C and f represent the equivalent capacitance and output frequency of the piezoelectric device 3, respectively. Furthermore, the level Vin of the input signal of the operational amplifier 1 is lower than the level Vs of the output signal of the piezoelectric device 3 by a predetermined coefficient. The coefficient depends on the equivalent capacitance C and output frequency f of the piezoelectric device 3 and the resistance value R of the third resistor 2R. In particular, the coefficient decreases with decrease of the output frequency f since the internal impedance of the piezoelectric device 3 increases. Accordingly, the amplification device 100 offers a lower gain. For example, when the output frequency f is equal to the threshold value f0=1/(2πRC), the level Vin of the input signal of the operational amplifier 1 is lower by 3 dB than the level Vs of the output signal of the piezoelectric device 3.
It is desirable to reduce the threshold value f0 for the output frequency f of the piezoelectric device 3 as much as possible in order to maintain the gain of the amplification device 100 high enough even in low frequencies. This requires the product of the resistance value R of the third resistor 2R and the equivalent capacitance C of the piezoelectric device 3 to be large enough. For example, when the equivalent capacitance C of the piezoelectric device 3 is of the order of 10 pF, the resistance value R of the third resistor 2R is required to be 1.6 GΩ or more for the 10 Hz or less threshold value f0 of the output frequency f.
It is desirable for miniaturization that the amplification device 100 is configured as a single integrated circuit. In that case, however, the third resistor 2R must be added externally as a discrete element separate from the other elements. There is an upper limit for sheet resistances, which is usually of a few tens of kilo ohms, due to a limit of control over impurity concentration. If a resistance element of 1.6 GΩ were configured in the region with a sheet resistance of 10 GΩ, for example, the aspect ratio L/W (the ratio of the length L to width W of the pattern) of the resistance element would reach 160,000. In other words, the length L would reach 160 mm even if the width W were 1 μm. Embedded resistance element of such a huge size would not be realistic. Accordingly, the third resistor 2R cannot be embedded at least on the substrate where the other elements inside the amplification device 100 are embedded. Therefore, for the conventional amplification device 100, miniaturization by further higher integration is difficult.
Integrated circuits are susceptible to adverse influences of the surrounding electrostatic discharge (ESD) through terminals connected to the outside. ESD protection circuits are, in general, provided around the pads for the purpose of the protection against the adverse effects of ESD, especially the prevention of the element destruction. The ESD protection circuits include diodes or transistors. The diodes and transistors are maintained in the OFF states under normal conditions, and on the other hand, turned on at the occurrence of an ESD surge, then connecting the pads to the ground or the power supply. Since the surge energy is absorbed into the ground conductor or the power supply, the adverse effect of the surge on the other elements is avoided.
The ESD protection circuit mounted on the substrate includes a reverse-biased PN junction. The PN junction of a larger area has a larger capacity for the surge currents, and thus provides higher reliability for the ESD protection circuit. However, a minute leak current flows in the reverse-biased PN junction. The leak current increases in proportion to the PN junction area, and in addition, drastically increases with temperature rise; for example, the amount of the leak current increases by nearly one order of magnitude for every 25 to 30 degree in temperature. In a thermal design for a semiconductor integration circuit, for example, heat generation of amplification devices or the like is considered, and accordingly, the guaranteed maximum value of the substrate temperature is set at a value, usually 125° C. or 150° C., sufficiently higher than the upper limit of a desired, guaranteed temperature range, usually of the order of 75 to 80° C. When the substrate temperature rises from 25° C. to 125° C., for example, the leak current increases by 1,000–10,000 times. In a PN junction designed for a area practical for an ESD protection device, the amount of the leak current is of the order of 10 pA at normal temperatures, and reaches 100 nA at approximately 125° C. When the leak current flows in the third resistor 2R, a voltage drop occurs. In the case of the resistance value of the third resistor 2R is 1.6 GΩ, the voltage drop reaches 16 V even if the amount of the leak current is 10 nA. On the other hand, a line voltage used for a miniature vibration sensor is restricted usually to 5V or less, and hence, the upper limit of the input dynamic range of the amplification device 100 is restricted to the order of 5V. Accordingly, the variation of the bias voltage due to the above-described voltage drop is excessive for the input dynamic range of the amplification device 100. Such an excessive variation of the bias voltage is undesirable since it can interfere with the operation of the amplification device 100, and further, it can cause the potential of the output terminal OUT of the amplification device 100, that is, the bias voltage for the next-stage amplification device to vary excessively. Accordingly, severe restrictions can be imposed on the design of the following amplification devices. Therefore, it is difficult to further improve the reliability of the whole of the multistage amplification device.